People have already tried contra-rotating propellers for paramotors:
But apparently previous attempts were abandoned because of things like throttle response lag:
However electric motors are different than 2-stroke engines, having a flat torque curve. That could make them very useful for contra-rotating propellers.
Here’s a workbench demonstration of a simple contra-rotating setup using electric motors:
Now that electric paramotors are becoming a thing, why shouldn’t contra-rotating propellers become more accepted too?